In the pre-cast concrete manufacturing industry, concrete members are often pre-made off site in casting yards or factories and then transported to site for erection as required. In a typical casting yard, concrete members are constructed on a steel bed. The advantage of using a steel bed is that the members can be constructed to a high degree of accuracy thus leaving an accurate finish on that surface of the concrete member in contact with the steel bed.
Sideforms are used to define the dimensions of the concrete members. Traditionally the sideforms are screwed or bolted to the steel bed. Once the concrete has been poured and allowed to cure, the screws/bolts and sideforms are removed. The cast concrete members are then lifted from the bed and the process repeated to form another member. However concrete members have become increasingly architectural having differing sizes and shapes. Therefore, if the concrete area of the new member to be cast is larger than the area of the previous member then the holes in the steel bed have to be patched so that the hole does not form an imprint in the next concrete member to be cast. Patching is often performed by welding the bolt holes then grinding them flush with the steel bed. However welding of the holes warps the steel beds as a result of the heat expanding the metal and this causes the steel beds to buckle and bow locally leaving imperfections in the surface of the concrete member. Moreover, this process is particularly labour intensive as the steel beds constantly require repair.
Other means of patching involve plugging the hole with a steel plug or cone and then grinding it flush with the bed. However forcing the plugs into the holes is found to cause a depression in the bed in the locality of the plug causing imperfections in the surface of the steel bed. Once again, the imperfection may form an imprint in the surface of the concrete member being cast. The grinder blades used to remove excess material from the plug also wear down the surface of the steel bed causing depressions in the bed's surface which again adversely affects surface of the concrete member being cast.
Still further means of patching involve plugging the hole with a plastic plug or cone and then grinding it flush with the bed. However it has been found that plastic plugs do not expand and contract at the same rate as the steel beds and do not give as good a finish, generally leaving either a protrusion or depression which is transferred to the surface of the concrete member.
More recently pre-casters have converted to using magnets to reduce the above-described damage.
The simplest form of precast magnetic clamp has an exposed magnetic pack and lever to engage and disengage the magnetic pack from a steel bed. The packs are placed in position on the steel bed and the sideforms placed against them, following which the sideform is attached to the magnetic pack by steel plates and screws. These packs are permanently magnetic and as soon as they are brought near the steel bed surface they exert a substantial amount of magnetic pull on the bed thus making it extremely difficult to position the magnets accurately. Once they engage they are difficult to move and adjust. They are unsafe to use as they can readily and easily clamp over limbs caught between the surface of the steel bed and the magnetic pack. To disengage the magnetic pack there is a lever on one or both sides of the pack that physically pushes the magnetic pack away from the steel bed so as to break the magnetic bond with the bed. The pack is physically pulled away from the steel bed by hand until such time as it is far away enough for the magnetic field not to have any substantial influence between the magnetic pack and bed. These magnetic clamps inhibit an operator from making simple and easy adjustments to the position of the sideform once the magnetic pack is engaged, aside from using a heavy object such as a mallet to manoeuvre the magnetic pack into position by force.
A second form of precast magnetic clamp has an exposed magnetic pack and a screw-down pin engagement/disengagement mechanism. These magnetic clamps differ in that rather than being separated from the steel bed via a lever of some sort they are separated from the steel bed via a threaded pin running through the magnetic pack from top to bottom. As the threaded bolt or pin is turned down into the magnetic pack the pin extends out through the bottom of the magnetic pack past the bottom face thus pushing the magnetic pack away from the steel bed breaking the magnetic bond and allowing the magnetic pack to be lifted from the bed.
A third form of precast magnetic clamp has an exposed plastic magnetic pack and operates either via a side lever action disengagement mechanism or a screw down pin disengagement mechanism. Instead of a lever used to push one end of the magnet up from the steel bed a threaded bar is located in the magnet body. When the threaded bar is screwed into the magnet body it protrudes past a bottom face of the magnet thus pushing the magnet body up and away from the steel bed.
The magnetic clamps with the screw down pins or threaded bars have the same drawbacks as the previously described magnetic pack magnets in that the operator still cannot make any adjustments to the position of the magnet and sideform after the magnet is placed on the steel bed. They are also very slow and cumbersome to use and the threads are subject to getting clogged with concrete thus making them inoperable.
A fourth form of precast magnetic clamp comprises a magnetic pack located within a housing with the magnetic pack moving vertically within the housing via either a screw mechanism or lever action. In use these clamps are able to be attached to the sideform and then engaged to the steel bed by moving the magnetic pack down through the housing on to the bed via either screws or a lever. The screw action is slow and cumbersome and prone to fouling of the thread by concrete. The same happens for the lever action as well as requiring the operator to constantly rely on and carry a long lever so as to give the operator enough leverage to pull the magnetic pack away from the steel bed.
A fifth form of precast magnetic clamp comprises a magnetic pack located within an open split housing where the magnetic pack is permanently fixed to the internal section of the open housing and then this internal section moves up and down within the external section of the housing. The housing is basically open in the sense that it only has sides, hence it has an open top and an open bottom. A plate containing the magnetic pack is hinged at the front of the magnet and simply drops down through the housing to allow the magnetic pack to attach to the steel bed. These magnets are cumbersome to use in that an operator cannot have the magnet attached to the sideform and make adjustments to the sideform for the magnet needs to be attached to position the sideform. Another problem is that a very long lever bar is required to disengage the magnet from the steel bed. Whilst levering the magnet from the bed, due to the excessive applied, leverage force the magnets tend to jump up during disengagement. Moreover, the hinge joint at the front wears causing the magnet to engage very quickly to the steel bed causing major safety issues.